1. Field of the Invention
The present invention relates to a method of completing a wellbore and, more particularly, to such a method wherein a spacer fluid is introduced into a wellbore and then a completion fluid to displace the spacer fluid and the spacer fluid is a low density fluid.
2. Setting of the Invention
In rotary drilling of wells, a drilling fluid is usually circulated down the drill string and back up the annulus between the drill string and the wellbore face. The drilling fluid can contain many different chemicals, but will most often contain a viscosifier, such as bentonite. When a casing or tubing string is to be cemented into the wellbore, any drilling fluid and remnants of the viscosifier present in the wellbore are preferably removed to aid the bonding of the cement between the casing or tubing string and the wellbore. In removing this drilling fluid from the wellbore and to clean the annulus, a wash or spacer fluid can be introduced ahead of the cement slurry, as is described in "Cementing Spacers and Washes Improve Production" by T. J. Griffin, et al., Oil and Gas Journal, Sept. 12, 1977.
The spacer fluids can contain: a weighting material, such as barite, to increase its density for better cleaning, as is described in U.S. Pat. No. 4,141,843 to Watson; a tar-like material or other oil-based materials, as described in U.S. Pat. No. 3,688,845 to Messenger for compatibility with wellbore fluids; and/or abrasive particles, such as sand, for cleaning the annulus and removal of unwanted material from the wellbore. These additional materials all can significantly increase the density of the spacer fluid, which is not always desirable.
One situation where the density of a spacer fluid is not desired to be increased is when a wellbore is completed through a highly porous and/or fractured formation. In this case, lost circulation material is usually added to the spacer fluid and/or cement slurry to prevent the introduced fluids from being lost into the porous formation. The use of lost circulation material is usually not desired because it is difficult to clean out of the wellbore prior to cementing and, if added to the cement slurry, can adversely effect the cement quality. Also, these highly porous formations are not capable of withstanding without fracturing the hydrostatic head in the wellbore of the spacer fluid, the wellbore fluids, and a completion fluid, such as a cement slurry. Obviously, the addition of lost circulation material will not prevent the opening of these formation fractures. A completion method is needed which includes a way to decrease the hydrostatic head of the fluids in the wellbore to reduce the likelihood of and to prevent the fracturing of the porous formations.
A method has been developed to decrease the hydrostatic head of fluids in a wellbore by reducing the density of certain fluids, including the spacer fluids and/or the cement slurries, and uses nitrogen gas pumped into the fluid to reduce the density thereof. The reduced density of the fluids thereby reduces the hydrostatic head of fluids in the wellbore to reduce the likelihood of fracturing the porous formation. This method has serious drawbacks, however, including the aspect that nitrogen can be difficult to use effectively in the field because the nitrogen gas separates out from the fluids at the pressures needed to be used in the wellbore and, when the gas separates it forms a gaseous bubble within the wellbore which can cause severe completion problems. Also, additional wellhead equipment is needed for the injection of the gas and a backpressure in the annulus is required to keep the gas in suspension in the fluids. This backpressure, which can be about 600-800 psi, is undesirable because all efforts need to be made to reduce, not add to, the pressure of the fluids on the formation.